The visual system of the fruitfly, Drosophila, offers a unique opportunity to examine the cellular and molecular events underlying axon outgrowth, pathfinding and target cell recognition. The developing retina produces three major types of photoreceptor neurons in a well-defined pattern of differentiation. Their axonal growth cones traverse long distances and make a number of stereotypic pathfinding decisions en route to specific target destinations within the developing optic ganglia of the brain. Within these ganglion cell layers, photoreceptor axons connect to target neurons in a precise retinotopic order. We plan to use the powerful genetic, cellular, and molecular methods available in Drosophila to understand how this retinotopic connection pattern is established. Our preliminary studies have implicated positional guidance cues in retinal axon guidance. These studies reveal features of retinotopic pattern formation in the Drosophila visual system that are reminiscent of the retinotectal system long studied in lower vertebrates. In addition, we have isolated mutations in ten genes that are required for retinal axon guidance. Here we propose experiments aimed at further characterizing the cellular and developmental functions of these genes and the biochemical functions of their products. Mosaic analysis will be used to identify the cells in which these gene functions are required. Selected genetic loci will be isolated and characterized at the molecular level. We also propose a second approach to the isolation of retinal axon guidance mutants that will permit the recovery of mutations which result in lethal defects in embryonic nervous system development. Finally, we have devised a method that employs the conditional expression of a toxin gene to ablate specific cells during development. This method will be used to examine the roles of various cell populations in the developing brain in providing guidance cues to retinal axons. It is likely that these studies in Drosophila will yield important insights into how the precise pattern of neuronal connections between the eye and brain is established in the visual systems of vertebrates, including humans.